The mga 3d driver shipped with XFree86 4 misbehaves in a variety of ways. Opening too many instances of 'gears' from xscreensavers (five or six) causes the Xserver to lock solid. Running ssystem windowed and occluded by other windows will also cause an Xserver hang in short order. The 3d waveform display plugin for XMMS can also cause the Xserver to hang if its window is resized or moved around too much. Most of these crashes are difficult to reproduce quickly, but they always occur. Strangely, quake3 seems to either run well for hours, or reboot the machine instantly, depending on what I've run recently. If the machine doesn't crash completely, the mouse cursor also still tracks, so I can kill the Xserver, but vesafb is trashed. Remote gdb indicates most of the crashes happen behind an int 0x80, so I suspect this may be a kernel issue.
K6-2/400, Matrox mgag200 AGP, stock RH7.0 install
I reported several bugs similar to these on the dri bugs list (sourceforge), but they don't seem to be keeping up with their bug reports.